Some might say that the latter (Icon= with a hardcoded path) is actually
harmful because it'll probably prevent icon theming from working for
this particular icon. Not that it'd be a big deal right now, but in
principle.

I've reverted back to the relative path now. I did the edit to use
absolute paths because the NewPackageProcess Wiki page refers to the
logjam package as an example, and the desktop file in that package uses
absolute paths; I thought it might be a portability thing.

I've also added a call to update-desktop-database in the %post and
%postun scripts, and added Requires(post) and Requires(postun)
dependencies on desktop-file-utils accordingly.